Parabolic reflector with rim of absorbing material to attenuate side lobes



Aug. 20, 1963 J. A. FENLON. JR 3,101,473

PARABOLIC REFLECTOR WITH RIM 0F ABSORBING MATERIAL TO ATTENUATE SIDELOBES Filed April 14, 1960 FIG. I Pr/or. AH

IN VEN TOR.

JOSEPH A. FENLON JR.

United States Patent M PARABGLiC REFLEQTGR WITH RIM 0F ABS DRB- ENGMATERIAL Ti) ATTENUATE SEDE LOBEF; Joseph A. Fenian, in, University(Z-ity, Mo, assignor to Mcfionneli Aircraft Qorporation, St. Louis, Mo.,a

corporation of Maryland Filed Apr. 14, i960, Ser. No. 22,210 2 Claims.(Cl. 343-750) The present invention relates generally to devices forshaping the radiation pattern of an antenna and more particularly toradar antenna radiation pattern shaping means.

. Various means have been devised in the past to eliminate or at leastto reduce antenna radiation in a particular direction. The known meanshave included radiation absorbing devices which are mounted adjacent tothe antenna to absorb radiation in a chosen direction. the knownconstruct-ions the radiation absorbing material has been fixedlyattached to an immovable object adjacent to the antenna such as a radomebut has not been satisfactory for use with moving antenna structuressuch as the antenna structures employed with radar gear because movementof the antenna changes the relative position of the absorbing materialwith respect to the antenna and thereby changes the direction or maximumradiation absorption relative to the radiation pattern. if the antennais moved through a large enough angle the main antenna beam may actuallybe directed at the absorbing material and be undesirably attenuatedwhile the side radiation which it is intended to be absorbed isunaffected. Another major problem of known radiation absorbing elementshas been their inability to withstand intense radiation resulting in afunctional breakdown when located in a strong electromagnetic field.These and other disadvantages of the known devices are overcome by thepresent device which comprises an antenna located adjacent to saidradiator, and means mounted on the antenna structure for attenuatingradiation therefrom in a .chosen direction relative to the antenna.

It is the principal object of the present invention therefore to providemeans for reducing antenna radiation in a chosen direction, such as theside lobes of the radiation pattern so that the etiective radiation isconcentrated as desired.

Another object is to uniformly reduce antenna radiation in a chosendirection relative to the antenna regardless of the direction in whichthe antenna is facing.

Another object is to maintain a constant relationship between a mainantenna radiation beam and a secondary or side lobe.

Another object is to provide an inexpensive, lightweight and compactdevice capable of absorbing antenna radiation, which device is capableof withstanding large amounts of radiation without breaking down andwhich can be installed on new or existing antenna structures withminantennae and the like by maintaining a constant fixed relationshipbetween the magnitude of the principal radiation lobe and the sidelobes.

These and other objects and advantages of the present invention willbecome apparent after considering the following detailed specificationin conjunction with the accompanying drawing.

Patented Aug. 20, 1963 In the drawing:

FIG. 1 is a longitudinal sectional view of an antenna mounted in aradome for movement about different axes relative to radiation absorbingmeans fixed in the radome which is representative of the prior art;

FIG. 2 is a cross-sectional view of an antenna equipped with radiationabsorbing means which moves with the movable portion of the antennaassembly; and

FIG. 3 is a view similar to FIG. 2, but illustrating the application ofthe radiation absorbing means to a particular zone.

Referring to the drawing more particularly by reference numbers, thenumber lb refers to an antenna assembly constructed according to theteachings of the prior art.

The antenna it has a parabolic reflector or shield 12, a radiator 14mounted centrally adjacent the concave side of the reflector 12, andsuitable leads l6 connecting the 'adiator id to a radar set 18 orsimilar radiation transmitting device. The reflector 12 is movablycarried on a universally pivoting device A.

When energy is fed to the radiator 14 from the radar set 13 it isradiated therefrom in a pattern, the characteristics of which dependupon the characteristics of the antenna components including theradiator and the reflector, the characteristics of the signal itself,and also the characteristics of the surrounding structures and theirlocation relative to the antenna.

In FIG. 1 the pattern is assumed to consist of a main or forwardradiation beam 29 which extends straight out from the antenna, and sidelobes which are contained in an annular envelope shown at 22. In themore generalized situation there will be a number of side lobes, but forthe sake of this invention these side lobes are dealt with as onegeneral side lobe 22, which forms the envelope of all side lobes.

The prior art involved attenuating the side lobe 22 in a local area, sayon the down side, by fixing in the radome Z4 absorbing material 26 whichattenuated the radiations passing through the same, thereby reducing theeffect on the scope pattern of the reflection from the ground or objectat the chosen side of the antenna. The difiiculty with this system isthat when the reflector 12 moves to face the material 26 the main lobe24D is attenuated which is undesirable. When the reflector moves to faceaway from the material 2.6 the side lobe attenuation is meduced oreliminated. In order to overcome the difiiculty with prior art devicesand to be able only to reduce the magnitude of the side lobe in a givendirection or portion of the annular envelope 22 depicted in FIG. 1,radiation absorbing material 26a is mounted on the reflector 12 as shownin MG. 2, or as shown at 2612 in FIG. 3. Any suitable radiationabsorbing material can be used for this purpose provided it canwithstand the intensity of the radiation without breaking down. i-leretofore, the absorber material of FIGSd, 2 and 3 consisted of a section offoam rubber with graphite therein and having a ply of aluminum foil onone surface.

A particular application where the present device has been found usefulis in connection with the radar systems employed on airplanes. in suchdevices not only is it important to properly modify the radiationpattern in given directions but it is also important that the size andweight of the equipment be kept at a minimum. During a typical trackingoperation, for example, using an airplane radar set, the radar antennamain beam is directed to a remote airborne target. The main beam Zil'athus directed, is reflected from the target and picked up by the sameantenna. At the same time energy is also being radidated in the form ofside lobes 22a. When the attenuation material 26a is extended about thelip' of the reflector i2 (HG. 2) the envelope of side. lobes 22a issubstantially attenuated in all directions. However, in FIG. 3

ddhiAYB 0 the envelope of side lobes is diiferently shaped at 22b fromthe lobe at 220, since part of the side lobe energy is radiateddownwardly and reflected from the car-th s surface. The reflected energyreceived from the target area, and from the earths surface as a resultof the side lobe energy, is fed to the radar set 18 and their presencemade known in the form of images on a radar scope. The radar scope isusually calibrated in azimuth and range.

The pictorial representations on the scope appear as line which isreferred to as the altitude line and which extends across the face ofthe scope as if it were a target at a range corresponding to theaircraft altitude, and as a target or bright spot. As the aircraftcloses on the target, the range to the target decreases and the targetmust pass through the altitude line. It is important to proper trackingthat the intensity of the power reflected from the target and receivedby the radar antenna be of much higher magnitude than the power theantenna receives which has been reflected from the earth. Thisdiiierence in intensity will insure that the radar remains locked on thetarget as the target passes through the altitude line. In this regard itshould be noted that the power due to noise and earth reflect-ions isdue inlarge part to the strength of the side lobe reflections. If thereflected energy due to the side lobes is large, the power whichcontributes tothe altitude line will be large and it will be morediflicult for the tracking radar to distinguish between the altitudeline and. target and lock on may be lost as the target passes throughthe altitude line. not impossible to successfully track.

As previously noted, to insure satisfactory tracking, the power receivedfrom the target signal must be appreciably greater than the powerreceived from the earths signal. The ratio of the power received fromthe earth and the power received from the target depends on thereflecting areas of the reflecting bodies and the gain of the antennawhen power is received from a particular direction. The ratio of theobserved area of the earth to the observed target area is a factor whichcannot be controlled. Therefore, in order to have an appreciablystronger signal from the target than from the earth, some means must bedevised to reduce the amount of energy radiated in and received from theside lobes without reducing the energy directed by the main beam at theabsorbing material so that the apparatus might produce results exactlyopposite from the desired result, or having little or no eflect at all.

The present invention overcomes these disadvantages by mountingabsorbing or attenuating material 26a on the antenna reflector 12 sothat it moves with the reflector and not relative thereto. Therefore,regardless of the attitude of the airplane or the direction in which theantenna reflector is facing, the reflector never faces toward or awayfrom the absorbing material. The purpose of applicants device then is toattenuate the side lobe power that otherwise would be radiatedtoward theearth or some other When this condition exists it becomes difficult if4. body so that the reflected energy received by the main lobe willvalways be substantially greater than the energy received from the sidelobes.

While the present invention has been described in connection with itsbroad application (FIG. 3) and in a specific antenna construction (FIG.2), it is obvious that it can also be used with many other antennastructures and for many other applications without departing from thespirit and scope of the invention.

1 claim:

1. For use with an aircraft radar-set having a scope, an antennacomprising a substantially round parabolic concave reflector having aforwardly extending central axis and a peripheral edge, a radiatorelement, meanssecuring said radiator element to said reflector andforwardly of said peripheral edge of said reflector on said axis, saidradiator in combination with said reflector being capable of producing aforward energy lobe orientated on the axis of said reflector and sidelobe energy orientated adjacent to the peripheral edge of said reflectorand extending generally radially of said axis, an elongated arcuatestrip of radiation absorbing material curved substantially the samedegree as said peripheral edge, said strip having an axially inner edgeportion telescopically received within the peripheral edge of saidreflector, means fixedly securing said edge portion of said strip to theperipheral edge of said reflector, said strip being flared in relationto said axis so that said edge portion fits flush against and parallelto the peripheral edge of said reflector, the remainder of said stripflaring radially outwardly and axially torwrdly from said reflector soas to be in radial alignment with said radiator whereby the side energylobe passes through said strip and is attenuated thereby, said stripattenuating said side lobe energy to a degree whereby when said forwardenergy lobe and said side lobe energy are reflected from an airbornetarget and earth respectively, images of substantially the samebrightness are produced on said scope, universal joint means connectedto said reflector and adapted to be secured to a support,

said universal joint means permitting said reflector, radiator andabsorbing strip of material to be moved as a' unit in any directionrelative to the base whereby the absorbing strip of material is alwaysin the path of said side lobe energy and never in the path of saidforward energy lobe.

2. An antenna as defined in claim 1, wherein said universal joint meansincludes a first pivot means having an axis perpendicular to andextending through the antenna axis, and a second pivot means having anaxis intersecting and perpendicular to both said antenna axis and saidfirst axis. l i s I References Cited in the file of this patent UNITEDSTATES PATENTS Germany Jan. 8, 1959

1. FOR USE WITH AN AIRCRAFT RADAR SET HAVING A SCOPE, AN ANTENNACOMPRISING A SUBSTANTIALLY ROUND PARABOLIC CONCAVE REFLECTOR HAVING AFORWARDLY EXTENDING CENTRAL AXIS AND A PERIPHERAL EDGE, A RADIATORELEMENT, MEANS SECURING SAID RADIATOR ELEMENT TO SAID REFLECTOR ANDFORWARDLY OF SAID PERIPHERAL EDGE OF SAID REFLECTOR ON SAID AXIS, SAIDRADIATOR IN COMBINATION WITH SAID REFLECTOR BEING CAPABLE OF PRODUCING AFORWARD ENERGY LOBE ORIENTATED ON THE AXIS OF SAID REFLECTOR AND SIDELOBE ENERGY ORIENTATED ADJACENT TO THE PERIPHERAL EDGE OF SAID REFLECTORAND EXTENDING GENERALLY RADIALLY OF SAID AXIS, AN ELONGATED ARCUATESTRIP OF RADIATION ABSORBING MATERIAL CURVED SUBSTANTIALLY THE SAMEDEGREE AS SAID PERIPHERAL EDGE, SAID STRIP HAVING AN AXIALLY INNER EDGEPORTION TELESCOPICALLY RECEIVED WITHIN THE PERIPHERAL EDGE OF SAIDREFLECTOR, MEANS FIXEDLY SECURING SAID EDGE PORTION OF SAID STRIP TO THEPERIPHERAL EDGE OF SAID REFLECTOR, SAID STRIP BEING FLARED IN RELATIONTO SAID AXIS SO THAT SAID EDGE PORTION FITS FLUSH AGAINST AND PARALLELTO THE PERIPHERAL EDGE OF SAID REFLECTOR, THE REMAINDER OF SAID STRIPFLARING RADIALLY OUTWARDLY AND